Conventionally, as an optical element for correcting refractive abnormalities in the optical system of the human eye, or as a substitute optical element after removal of an crystalline lens, etc., an ophthalmic lens has been used. As specific ophthalmic lenses, in addition to eyeglasses lenses, there are contact lenses that are overlapped on the cornea, or ophthalmic lenses mounted directly in the human eye, such as an intraocular lens (IOL) used by being inserted intracapsularly in place of the intraocular crystalline lens, or a phakic intraocular lens (ICL) used by being inserted in the anterior chamber of the intraocular crystalline lens, etc., and these are widely used because they provide a wide field of view as well as reduce a sense of discomfort of vision.
However, in recent years, there is an increase in people continuing to use contact lenses when they have reached the age of having presbyopia. For people with presbyopia, their accommodation power decreases, so there is a symptom of having difficulty in focusing on nearby objects. Thus, multi-focal contact lenses which can also focus on nearby objects are needed for presbyopia patients. Also, for patients who have undergone cataract surgery, since the crystalline lens which is in charge of the accommodation function is removed, even if an intraocular lens is inserted as a substitute, the symptom of having difficulty in seeing close up remains. With the intraocular lens as well, it is necessary to have a multi-focal function having a plurality of focal points. In this way, the need for multi-focal ophthalmic lenses to reflect the aging society has increased even further in recent years.
As a method for realizing this multi-focal ophthalmic lens, known examples include a refractive multi-focal ophthalmic lens that forms a plurality of focal points based on the principle of refraction, and a diffractive multi-focal ophthalmic lens that forms a plurality of focal points based on the principle of diffraction. With the latter diffractive ophthalmic lens, provided are a plurality of diffractive structures which are formed in concentric circle form on the optical part of the lens, and a plurality of focal points are given by the mutual interference effect of light waves that pass through the plurality of diffractive structures (zones). Therefore, compared to a refractive lens that gives focal points using the refractive effect of light waves at the refracting surface comprising boundary surfaces with different refractive indexes, with the diffractive type multi-focal ophthalmic lens, there are advantages of being able to set a high lens power while inhibiting an increase in lens thickness, etc.
Generally, a diffractive multi-focal lens has a diffractive structure for which the diffractive zone pitches become gradually narrower toward the periphery from the lens center according to a rule called the Fresnel pitch, and multiple focal points are made by using the 0th order diffracted light and +1st order diffracted light generated from that structure. Normally, the 0th order diffracted light is used as the focal point for far vision, and +1st order diffracted light is used as the focal point for near vision. Using this diffracted light distribution, it is possible to make a bifocal lens having both far and near focal points.
Also, as in Japanese Unexamined Patent Publication No. JP-A-2010-158315 (Patent Document 1) disclosed by the present applicant, or in PCT Japanese Translation Patent Publication No. JP-A-2013-517822 (Patent Document 2), known are items for which the number of focal points are further increased, and as a result, it is possible to set focal points for intermediate vision in addition to those for far vision and for near vision.
Furthermore, in PCT Application No. PCT/JP2014/071113 (Patent Document 3), the present applicant proposed a diffractive multi-focal lens with improved degree of freedom of the focal point setting position for intermediate vision. The diffractive multi-focal lens of this earlier application is a diffractive multi-focal lens having a diffractive structure comprising a plurality of zones in a concentric circle form, characterized in that: the diffractive structure includes an overlapping region for which at least two zone profiles are overlapped on the same region in at least a portion thereof; and at the overlapping region, at least a portion of a first zone profile of the at least two zone profiles has a zone pitch expressed by Equation 1, and at least a portion of a second zone profile of the at least two zone profiles has a zone pitch expressed by Equation 2, and an addition power P1 given by the first zone profile and an addition power P2 given by the second zone profile are determined by a relational expression of Equation 3, where a and b are mutually different real numbers, and a value of a/b is a value that cannot be expressed by a natural number X or by 1/X.
                              r          n                =                                            r              1              2                        +                                          2                ⁢                                                                  ⁢                                  λ                  ⁡                                      (                                          n                      -                      1                                        )                                                                              P                1                                                                        Equation        ⁢                                  ⁢        1            λ: Design wavelengthrn: nth zone radius of the first zone profiler1: First zone radius of the first zone profileP1: Addition power of the first zone profilen: Natural number
                              r          m                =                                            r              1                              ′                ⁢                                                                  ⁢                2                                      +                                          2                ⁢                                                                  ⁢                                  λ                  ⁡                                      (                                          m                      -                      1                                        )                                                                              P                2                                                                        Equation        ⁢                                  ⁢        2            λ: Design wavelengthrm: mth zone radius of the second zone profiler1′: First zone radius of the second zone profileP2: Addition power of the second zone profilem: Natural number
                              P          2                =                              a            b                    ×                      P            1                                              Equation        ⁢                                  ⁢        3            
Also, with the diffractive multi-focal lens noted in Patent Document 3, for example in Equation 3, by setting a and b to be integers of zero or greater, an overlapped and synthesized profile has a repeated structure of periodic zones, and it is possible to more clearly realize the generation of at least three focal points over the entire area of the composite profile. Also, in this Equation 3, by setting a and b such that a/b>1/2, the focal point set at a position in the middle of far and near, can be set closer to the near focal point than the far focal point, and it is possible to set a focal point suitable for viewing a computer screen, for example.
However, with this kind of diffractive multi-focal lens, the existence of “halo” is pointed out as a problem particularly when used as an ophthalmic lens. “Halo” is a phenomenon of a band shaped or ring shaped light occurring around a light source when viewing a far light source at night, for example, and occurs particularly easily for point-shaped light sources such as a far street light or automobile headlight, bringing a decrease of visual acuity when using an ophthalmic lens at night.
In regards to the “halo” phenomenon, an explanation will be given with a specific example in the Embodiments for Carrying Out the Invention section described later, but as disclosed by the present inventor in Japanese Unexamined Patent Publication No. JP-A-2014-228660 (Patent Document 4) and International Publication No. WO2013/118176 (Patent Document 5), for example when focusing on the focal point for far vision, light from the far distance forms a main peak at the image plane center of the far focal point. Here, due to the fact that the lights intensified each other at other focal point positions, etc. also reach the image plane position of the far focal point, small peak groups caused by multi-order light exist around the main peak that forms the far focal point in the image plane of the far focal point, which conceivably cause halo. The intensity of this multi-order light is extremely small compared to the intensity of the main peak. However, when viewing an object in a dark environment or in a high contrast environment, this is thought to be recognized as a halo by being perceived by the retina with the high sensitivity of the human eye. Also, the problem of blurred vision having symptoms of vision as if being hazed when viewing an object or viewing an object in fog is also thought to be caused by the same mechanism as halo.